Making jam – the Science

If you have fruit trees or plants in your garden you’re probably also swamped with these fruits during certain times of the year. And who has enough space to store 5 kg of figs or 20kg of apples?

Nowawadays commercial farmers have luckily found a solution for this. Fruits can often be stored for a long period of time, or a longer period of time, through refrigeration and proper control of the composition of the air in which they are stored. However, damaged fruits still can’t be stored, nor can your 5 kg of home grown figs.

This problem isn’t new of course and for centuries, if not longer, people have found ways to preserve these valuable fruits. The most common way that you’ll see all around is, to make jams or other types of preserves.

What is a jam?

Jamming, the verb, means “to squeeze or pack tightly into a specified space”. This describes a fruit jam quite well, it is a lot of fruit jammed into a jar. Since the fruit has been boiled down, the original structure has broken down. It has lots its bite and has become a lot softer, no need to chew anymore. The two main ingredients of a jam are the fruits and sugar which is why a jam is sweet.

A jam is also quite dense, it won’t flow easily, it is very viscous, gel-like. In order to get that more viscous texture of a jam often pectin is added as well. However, not in all cases does this have to be added separately since various fruits already contain this pectin.

How is jam made?

Making jam always starts by boiling fruits. The heat will break down the structure of the fruit. The cell walls are broken down and the moisture from inside the fruit is released. Other structures inside the fruit also break down, causing the whole fruit to soften. The sugar in the jam is added at the start of the boiling process. The sugar will dissolve in the water that comes out of the fruits.

By boiling the fruits not only does the texture change, the water content is also reduced. This is very important for making the jam keep good for a long period of time. By boiling of the liquid you’re increasing the concentration of sugar and fruit components. The jam is finished when it’s become thick enough so it doesn’t flow by itself anymore. Jam at this point will have pretty complex flow properties.

The role of pectin in jam

You will often see pectin being added to the fruits when making a jam. Pectin is a large complex polysaccharide (a carbohydrate) and has the ability to form gels. The large and complex structure of pectins can prevent other molecules (e.g. water) from moving around freely. As a result, the water is more or less kept in place and a gel is formed.

Some recipes for making jam will call for pectin and a lot of supermarket jams contain pectin as well. This is fine, but often you can well do without the additional pectin. As a matter of fact, various fruits (e.g. apples and berries) contain plenty pectin themselves to form this thicker gel-like structure.

Pectins seems to prefer a slightly acidic environment, this is when they are at their best with regards to gelling. Adding a little bit of acidity, e.g. lemon juice, can therefore improve the effectiveness of the pectins.

The structural role of sugar

Despite pectin’s gelling properties, these do not tend to be strong enough to gel cooked fruits. This is where the sugar comes into play as well. A sugar syrup with only little water is already thick and viscous. It will not flow as freely. The sugar holds on to the water molecules. This effect, combined with the pectin, is what makes the jam become nice and thick.

If you’ve ever made a jam and boiled it for too long. That is, it didn’t burn, but it became way too thick, there is no reason to worry either. Just add some more water and bring to the boil again. The structure of a sugar syrup is reversible so you can just try again. Even better, if you’re using the same fruit type, you can use a thermometer to determine when the jam is ready. The boiling point of a sugar solution depends on the concentration of sugar. So once you know which consistency you’re looking for, take care to note the temperature. Next time you’re making the jam you will only have to boil up to that temperature and the jam will be ready.

making jam: fig and grape jam in bowls

Why jam is a way to preserve fruits

When discussing the shelf life of foods one term that comes back all the time is water activity. The water activity describes the amount of ‘available’ water in a food. Food with a high water activity (maximum value is 1,00) have a lot of available water. Fresh fruits have a high water activity.

Micro organisms that can spoil food also need a certain water activity to grow and survive. If the water activity is too low, the micro organisms cannot grow any more. By drying fruits the water activity can be lowered below this critical level, which allows them to be kept for a long period of time. Jam making is another way to lower the water activity. We both remove water during the boiling process, but what’s more, by adding a lot of sugar the amount of available water decreases even further. Sugar ‘binds’ this water, making it unavailable for micro organisms. The overall water activity will be low enough to make it very unattractive for micro organisms to grow.

On top of that, fruits tend to be slightly sour. This low pH is another hurdle for micro organisms since most micro organisms, especially those that make you sick, will not be able to grow under these acidic conditions. Last but not least, jams are generally packed in well cleaned or even sterilized bottles and cans. This means there aren’t even any micro organisms in there to grow.

But I’ve seen moulds grow on my jam

Despite all these hurdles that we’ve thrown up against the micro organisms, this often doesn’t mean the foods don’t spoil at all anymore. Once the clean bottle of jam has been opened and once you’ve put a dirty knife or spoon in the bottle, micro organisms have been reintroduced. This is why an opened pot of jam should be stored in the fridge. Micro organisms will be present, but they will grow very slowly (or in some cases, not at all) thanks to the sugar, low temperature and the slight acidity. However, several months after opening you might still see some growth of moulds, especially on areas that have been exposed to air that have become a little damp or moist.

Sugar-free jam

Making sugar free jam is possible, however, it isn’t really jam anymore. Without the sugar the shelf life of the jam will be a lot shorter, probably days instead of months. Also, the flavour will be completely different as will the texture. In the supermarket you will therefore tend to see jam with less sugar and not without sugar. These sugar reduced jams still tend to contain more than 30% sugar, so not exactly ‘light’. If you’d prefer jam without sugar a better option might be to look into a jelly or dried fruits.

Jam recipes

Making jam always consists of a very similar procedure, take fresh fruits and boil them with sugar. If you prefer a sweeter jam than the one below, just add some more sugar. Take care not to remove too much sugar though, as we discussed above, the jam might not keep as long anymore with less sugar.:


Making jam – the Science

  • Author: Science Chef
  • Prep Time: 10 mins
  • Cook Time: 90 mins
  • Total Time: 1 hour 40 mins
  • Yield: 2-3 pots 1x
  • Category: Condiment



  • 1 kg of fruits, skins removed and cut into smaller pieces
  • 500g of sugar

Fruit suggestions

  • Pineapple; one of our favorites for sure, remove the skin and cut into smaller pieces
  • Figs; use a spoon the remove the inside flesh from the skin
  • Grapes; add the whole graps, but make sure to sieve the mixture a few minutes after you start boiling to remove the seeds and skins, the skins aren’t too bad, but the seeds make the jam very unappealing.
  • Fresh berries; just add them whole, if there are a lot of seeds, sieve the mixture


  1. Cut the fruits into smaller pieces and place in a pot on the stove on a low heat.
  2. It’s best not to add any water at this point. However, if you need to because your fruits are caramelizing on the bottom (getting brown), add a very thin layer of water. The amount of water will increase by itself while the fruits are breaking down and releasing their moisture.
  3. Once the fruit has broken down well enough, you can sieve the mixture to remove seeds or larger particles. Not all fruits require sieving though.
  4. At this point the individual fruit pieces should have broken down and become nice and soft.
  5. Add the sugar and keep on boiling on a low heat. Do not put it on a high heat, it will start browning and sticking to the bottom of the pan. Stir regularly.
  6. Your jam is ready when it stays firm once it’s cooled down. Any easy way to test is to take a small bit out of the pan and leave it to cool to room temperature. If you measure the temperature at the moment it’s done you will know the next time for how long to cook the jam.


That’s it for the science basics of making jam. Hope you learned something and please let me know if you have further questions, I’d love to answer them!

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